Thirty to 50 percent of patients diagnosed with gastroesophageal reflux disease (GERD) are not satisfied with symptom control derived from proton pump inhibitors (PPI). This group of patients; PPI non-responders (PPI- NR), pursue further diagnostic procedures and interventions with low yield. This approach is problematic because symptoms are not related to pathologic reflux in at least 50% of PPI-NRs referred for ambulatory reflux testing and therefore it does not lead to improved outcomes or even inform us as to why a patient did not respond to PPI. The development of novel therapies for this group is limited by lack of agreement on the definition of a PPI responder and the lack of a model that accounts for the heterogeneity of continued symptom generation. Our goal is to validate a PPI-NR model that accounts for factors above and beyond pathologic reflux, reducing the need for low-yield ambulatory reflux monitoring and informing the development of novel treatments. Study 1 will provide an equation in which pre-test clinical and questionnaire data sufficiently enables probability estimation for each of 4 reflux phenotypes, derived from ambulatory reflux monitoring. Study 2 will yield a similar equation which will predict one's ability to withdraw PPI therapy when indicated.